Endoscopes are often used for medical examinations, interventions and analyses. Endoscopes with the smallest diameter possible are used in order to reach the examination or operation site passing through naturally existing body orifices or by means of little traumatic steps. Traditionally, such endoscopes are produced by bundles of optical fibers. Today, the CMOS sensor technology is increasingly used, wherein a miniaturized image sensor is placed directly on the distal end of the endoscope to transmit the image in the form of an electrical signal.
Image sensors for use on the distal end of medical endoscopes are mostly structured today in such a way that they can operate to a large extent autonomously and are connected to the proximal end only via a data line and the power supply. The addition of a clock line is generally not desirable because of the additional sensor contacts and additional signal lines required for the clock. This is why an oscillator, which defines the sensor clock autonomously, is integrated in such sensors. The simplest form uses a digital ring oscillator. Refer in particular to the article by M. Wäny et. al. SPIE in the magazine “Photonics West” of January 2009. Reference: E109-E1114-9_7249-32. This kind of embodiment has the disadvantage that the sensor clock is only determined approximately and is strongly influenced by fluctuations in the manufacturing process and by operating conditions such as e.g. the operating temperature.
The realization of frequency-stable oscillators, with a stability as it is known e.g. from quartz oscillators, is not possible purely in CMOS technology. The clock signal generated by such a purely CMOS oscillator shows large fluctuations compared with a clock signal generated by a quartz oscillator, especially when the sensor temperature varies or the power supply is subjected to fluctuations. In article M. Wäny et al. “Ultra small digital image sensor for endoscopic applications” Proc. of 2009 International Image Sensor Workshop, 26 Mar. 2009 (2009-03-26) XP55035306, Bergen, Norway, techniques are described in order to make a miniaturized image sensor more robust with respect to the influence on its functions, among others also with respect to the influence of the environmental conditions on its operating frequency. But possibilities or methods to adjust such an autonomously operating sensor to a clock set externally are missing. Capturing the images however often requires to operate the sensor synchronously with an external clock, to allow sending the image data without intermediate storage to a video output device such as e.g. a video monitor. There are also applications, e.g. the stereoscopic image capture, for which it is advantageous to operate a plurality of image sensors synchronously with respect to each other.
The international publication WO 2007/101360 A1 relates to a miniaturized image sensor for a smallest-size endoscope, which operates autonomously and generates its sensor clock itself. But the publication lacks methods or possibilities to operate several such image sensors synchronously with each other or such image sensor synchronously with a clock set externally (e.g. coming from a pulsed lighting).
The European patent application EP 2 108 943 A2 describes a device for fluorescence imaging including light generation means. However, the lighting sources described in this publication have to be synchronized by means of a control device with respect to the image frequency of the image sensor, there is no possibility to synchronize the image sensor with respect to the lighting frequency. This publication also mentions the use of a plurality of image sensors for capturing 3D image data, but there is no possibility to synchronize the image sensors with respect to each other. This will lead easily to artifacts and miscalculations, in particular when capturing 3D images with objects that move in the image, since the distance traveled by an object moving in the image during the time lag between the respective image captures by the two sensors will be considered wrongly as a depth information.